A. Field of the Invention
The present invention relates to a zoom lens system composed of two lens groups, and, more particularly to a zoom lens system in which inexpensive lens materials can be used, a good aberration performance is maintained, the location of image point does not change even though the atmospheric temperature changes, and a good optical performance is maintained.
B. Description of the Prior Art
Recently, so called "lens shutter" cameras have become compact, have automatic features, and have a zoom lens with a focal length which varies linearly. The lens shutter cameras are required to be compact, light, and inexpensive especially because they are for non-professional users.
Generally, zoom lenses for the lens shutter cameras are divided into a two lens group or a three lens group. The two lens group is preferred for a zoom ratio of about 2 times because a lens barrel for the two lens group can be designed and controlled easily and produced economically.
For example, Japanese Patent laid-open No. 1-193808 (title: zoom lens for compact cameras, published on Aug. 3, 1989), Japanese Patent laid-open No. 1-288823 (title: small-sized zoom lens, published on Nov. 21, 1989) and Japanese Patent laid-open No. 3-260610 (title: small-sized zoom lens, published on Nov. 20, 1991) disclose zoom lenses of the two lens group.
The zoom lens disclosed in Japanese Patent laid-open No. 1-193808 has a zoom ratio below 1.9 times and uses expensive lens materials. It is not economically successful because a plastic lens is used as a first lens and is susceptible to external impact. The zoom lens disclosed in Japanese Patent laid-open No. 1-288823 has many elements and uses aspheric surface lenses made of glass which are expensive. The zoom lens disclosed in Japanese Patent laid-open No. 3-260610 is made of a relatively inexpensive material. However it has a zoom ratio too low to maintain a good optical performance.